To the untrained eye, the L’Anse aux Meadows archeological site on the island of Newfoundland—since 1978, a UNESCO World Heritage Site—doesn’t look like much. The reconstructed Viking huts and workshops are quaintly photogenic, but they are, after all, reconstructions. All that remains of the original buildings are low protuberances in the surrounding fields: the remains of timber-and-turf structures, covered by lush grass. But the site, modest as it is, represents a key moment in history: It is proof that Europeans crossed the Atlantic some five centuries before Columbus. And while scholars continue to debate how far the Vikings voyaged or why they came to these shores in the first place, they now have a much better idea of when they were here, thanks to a new study that places the Vikings at this spot in AD 1021—exactly 1,000 years ago.

Though the L’Anse aux Meadows site had been studied since the 1960s, only rough estimates of its age had been possible until now. Radiocarbon dating, which was in its infancy when the site was first studied, yielded results with wide margins of error. But a new technique that leverages astrophysics in the aid of archeology has lent the process a far greater degree of precision.

The key to this technique is a “cosmic ray event,” a burst of energetic particles from space—likely from the sun—that struck Earth’s atmosphere in the late 10th century AD. “We think in 992, the sun sent out a big burst—either a solar flare or a coronal mass ejection—of highly energetic particles,” says Michael Dee, a geophysicist and archeologist at the University of Groningen in the Netherlands. When they hit our atmosphere, the barrage of particles triggered the production of carbon-14, a radioactive isotope of carbon, which was absorbed by plants around the world in the following year, AD 993.

Carbon-12, with six protons and six neutrons, accounts for about 99 percent of all the carbon on Earth; the slightly heavier carbon-13, with an extra neutron, accounts for about 1 percent. Carbon-14, which has two extra neutrons and is radioactive, occurs only in trace amounts, accounting for about one out of every trillion carbon atoms in the atmosphere. But the solar outburst caused carbon-14 levels to jump by about 12 percent, the authors say. Trees all over the world, if they were alive at that time, contain a ring documenting this carbon-14 spike. So if you’re lucky enough to find wood from a tree that was alive when one of these extreme solar storms happened, you just need to count outwards from the ring in which the spike was measured to the edge of the tree, to determine the date on which it was felled.

Reconstruction of a timber-and-sod Viking house at L’Anse aux Meadows National Historic Site in Newfoundland.

Photograph: Dan Falk

The objects Dee and his colleagues studied, recovered from L’Anse aux Meadows decades ago and carefully preserved in a freezer in a Parks Canada storage facility in Dartmouth, Nova Scotia, fit the bill perfectly. They include a tree stump that may have been pulled from the ground as the land around the Viking site was being cleared—and which, critically, still had its “bark edge” intact. Since there were 28 rings from the carbon-spike ring to the edge, the cutting of the tree can be pegged to AD 1021. (The fact that this is exactly 1,000 years ago is just a coincidence, though a welcome one, Dee says.)